In general, a wiper is installed on the windshield of a vehicle in order to overcome a visibility obstacle caused by rainwater in a rainfall. These wipers control an intermittent speed thereof step by step depending on a dropping degree of rainwater. However, since a speed control system of the wiper is controlled by only a few steps, the wiper may not be operated at a speed a driver desires depending on an amount of rainwater.
In order to overcome this problem, a circuit board on which a light source and a sensor which is a light receiving element are mounted is inclined with respect to a windshield surface, so that a rain sensing efficiency can be increased by receiving only an optical signal reflected from raindrops themselves while minimizing an influence of light reflected from the windshield surface. That is, light directly reflected from the windshield goes outside a light receiving range of the light receiving element and is reflected on the windshield to minimize an amount of light received by the light receiving element. On the other hand, since only the amount of light reflected by the raindrops is received by the light receiving element, the circuit board having the light source and the light receiving element is disposed at an angle to the windshield surface so as to detect only an irregular reflection signal from the raindrops.
However, even in the case of a product in which the light source and the light receiving element are arranged with the circuit board inclined with respect to the windshield surface of the vehicle, the light emitted from the light source may be directly absorbed by the light receiving element, and thus the rain sensing efficiency is somewhat incomplete and insufficient. That is, the light emitted from the light source spreads over a range of angles, and even if the light source and the light receiving element are arranged so as to be inclined with respect to the windshield surface, a part of the light is directly emitted toward the light receiving element other than the light coming out of the windshield. Thus, there is a problem of somewhat lowering the raindrop detection efficiency due to interference light absorbed by the light receiving element from the light source.
Even in the case of designing to minimize ambient interference light due to headlight of the around traveling vehicles as described above, interference light which may not inevitably be blocked is generated. A light sensing rain sensor itself is a very sensitive sensor product, so that a small amount of ambient light that may not be blocked inevitably affects the sensor, and thus it can not help but have a limit to have a highly accurate rain sensing effect. In addition, in order to realize a structure for minimizing the influence of the ambient light, it has no choice but to have a somewhat complicated structure, thereby inevitably having a limitation such as a somewhat inefficiency in productivity and a somewhat higher product cost.